1. Field of the Invention
The invention relates to a control apparatus for a vehicle, and a method of controlling a vehicle. More specifically, the invention relates to a technology in which a torque output from a power source is boosted during a torque phase when an automatic transmission upshifts.
2. Description of the Related Art
When an automatic transmission upshifts, a driving force decreases during a torque phase, and then the driving force increases and a shock occurs when the torque phase ends and an inertia phase starts. To suppress the shock when the automatic transmission shifts, the technology in which the torque is boosted during the torque phase is proposed.
Japanese Patent Application Publication No. 2004-316838 (JP-A-2004-316838) describes a shift control apparatus for an automatic transmission, which executes a torque-boost control to boost torque output from a power source so that the automatic transmission shifts in an expected manner. The shift control apparatus includes an upper limit calculation portion, a torque calculation portion, a target engagement pressure setting portion, a torque control portion, and a friction element control portion. When an instruction is provided to perform the gear shift, the upper limit calculation portion calculates the upper limit value of the torque that may be output from the power source during the gear shift. When the instruction is provided to perform the gear shift, the torque calculation portion calculates the torque to be output from the power source when the torque is boosted. When it is determined that the calculated torque exceeds the calculated upper limit value, the target engagement pressure setting portion sets a target engagement pressure for a friction element to be engaged during the gear shift, based on the calculated upper limit value. When it is determined that the calculated torque exceeds the calculated upper limit value, the torque control portion controls the power source so that the torque output from the power source is equal to the calculated upper limit value during the gear shift. When it is determined that the calculated torque exceeds the calculated upper limit value, the friction element control portion controls the friction element so that the engagement pressure is equal to the target engagement pressure during the gear shift. As the upper limit value of the torque decreases, the target engagement pressure (“shelf pressure” described below) decreases.
The shift control apparatus described in the above publication corrects the engagement pressure for the friction engagement element through a feed-forward control to compensate for insufficiency in torque boost because of the upper limit value. Because the engagement pressure for the friction element is corrected through the feed-forward control, it is possible to avoid a problem relating to slow response, which would arise if the engagement pressure is corrected through a feedback control. Thus, it is possible to reduce the amount by which the output torque changes immediately after the torque phase ends during the gear shift.
When the automatic transmission upshifts, the engagement pressure for the friction engagement element to be engaged is maintained at a predetermined pressure (shelf pressure) during the torque phase, and then the engagement pressure is increased at a predetermined gradient during an inertia phase. In the shift control apparatus described in the above publication, the engagement pressure is determined based on the torque input to the automatic transmission during the torque phase. When the torque-boost control is restricted during the torque phase, the engagement pressure during the torque phase is smaller than when the torque-boost control is not restricted during the torque phase. In other words, when the torque-boost control is not restricted during the torque phase, the engagement pressure during the torque phase is greater than when the torque-boost control is restricted during the torque phase. Thus, if the gradient of the engagement pressure during the inertia phase is set regardless of the torque-boost control during the torque phase, and the torque-boost control is not restricted during the torque phase, the engagement pressure is high at the start of the inertia phase, and therefore, the engagement pressure is high at the end of the inertia phase, that is, when the friction engagement element is engaged. This may cause a shock.